Compression ignition engines produce an exhaust emission that generally contains at least four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). Emissions control devices known as oxidation catalysts (or diesel oxidation catalysts) are commonly used to treat carbon monoxide (CO) and hydrocarbons (HCs), including the volatile organic fraction (VOF) of particulate matter (PM), in exhaust emissions produced by compression ignition engines. Such catalysts treat carbon monoxide (CO) by oxidising it to carbon dioxide (CO2), and treat hydrocarbons (HCs) by oxidising them to water (H2O) and carbon dioxide (CO2).
Oxidation catalysts for compression ignition engines typically comprise a platinum group metal (PGM) and a support material, which have been washcoated onto a substrate. A problem with such oxidation catalysts is that they take several minutes to heat up to their effective operating temperature from a cold start and in that time a significant amount of pollutant can be emitted into the air.
The effective operating temperature of an oxidation catalyst is often measured in terms of its “light-off” temperature. This is the temperature at which the catalyst starts to perform a particular catalytic reaction or performs that reaction to a certain level. Normally, “light-off” temperatures are given in terms of a specific level of conversion of a reactant, such as conversion of carbon monoxide. A T50 temperature is often quoted as a “light-off” temperature because it represents the lowest temperature at which a catalyst catalyses the conversion of a reactant at 50% efficiency.
One way of reducing or preventing the emission of pollutants that occurs shortly after the cold start of a compression ignition engine is to electrically heat the oxidation catalyst to rapidly bring it up to its “light-off” temperature, usually its CO and/or HC “light-off” temperature. However, there are many disadvantages associated with electrically heating an oxidation catalyst, such as the additional electrical demand placed on the engine/battery, the requirement to electrically insulate the substrate that is electrically heated and the space in the exhaust system needed therefor. When the substrate to be electrically heated is metallic, some oxidation catalyst compositions poorly adhere to the metallic substrate.